Water Signs

Each Mars Exploration Rover scans the land with a pair of panoramic cameras that detect visible light like human eyes do. But buried at the base of the rover’s mast lies a third eye, blind to light as we know it, but exquisitely sensitive to heat, or thermal-infrared radiation.

Every Martian rock, radiating heat absorbed from the sun, broadcasts its mineralogical composition. But who’s tuned in? An instrument called the Mini-Thermal Emission Spectrometer, or Mini-TES, can receive those broadcasts, loud and clear.

Unlike an eye or a camera, the Mini-TES has only a single detector. "It’s not an imaging spectrometer. We don’t have a two-dimensional detector array that can view the scene all at once like a snapshot. We actually have to build up that scene," says Steve Ruff, a member of the Mini-TES team and a faculty research associate at the Mars Space Flight Facility at Arizona State University.

The Mini-TES is a descendant of the Mars Global Surveyor’s Thermal Emission Spectrometer (TES) instrument and operates using the same principles. It also uses the same linear motor and shares some electronics, but the Mini-TES has been simplified and has laser diodes in places of the TES’s neon bulbs. The Thermal Emission Imaging System (THEMIS) carried on the Mars 2001 Odyssey spacecraft improved on TES by combining a visual imaging system with an infrared imaging system. All three instruments were designed to detect minerals that provide evidence of past water on Mars.

The scientists can make an image of sorts with data from Mini-TES, something like a mosaic made of many individual picture elements, or pixels. It’s a slow process, and the image itself is not the real goal.

"This process of building up a panorama of Mini-TES data is very time-consuming," Ruff says. The Mini-TES looks upward at an angled mirror like the mirror at the top of a periscope. Repeatedly rotating the mast and tilting the mirror enable the Mini-TES to assemble a full, panoramic view around the rover. But all motion must pause for a full two seconds for the Mini-TES to collect one pixel’s worth of data. It will take at least two days for the Mini-TES to complete one panoramic picture.

A Pixel Is Worth a Thousand Words

For all of its slowness, the Mini-TES packs a punch in each pixel. Where a digital photograph might have red, green, blue and brightness information in a single pixel, a Mini-TES pixel includes measurements of about 150 different "shades" of infrared light. The instrument is able to split out each of these wavelengths from the infrared spectrum that it can detect.

By analyzing a Mini-TES spectrum, the researchers can determine the mineral composition of a particular rock. Every mineral molecule vibrates at a distinctive frequency, emitting a particular wavelength of infrared light. "In these thermal infrared wavelengths," Ruff says, "all minerals have a distinctive infrared signature that’s really like a fingerprint." The ASU lab has built up a library of TES spectra using rocks whose mineral composition is known, Ruff says. "So when it comes time to interpret spectra from Mars, we can refer to these spectra in the library to understand what we’re seeing at Mars."

A single Mini-TES pixel a single spectrum may include one rock or part of a scene with many rocks. "It totally depends on the distance of that rock from the Pancam mast," Ruff says. "Rocks up close can definitely be bigger than the single pixel element of our instrument." That would allow the Mini-TES team to analyze a spectrum and to determine the composition of that specific rock.

"As you move out from the rover, you start to have this problem of multiple rocks within a single pixel, a single spectrum." The scientists can solve such puzzles using a combination of computer power and experienced eyes, a process they call deconvolution. If a spectrum contains an interesting rock somewhere within its field of view, the Mini-TES team will know. "We’re pretty good at deconvolving these spectral data to try to figure out what’s there, even in mixtures," Ruff says.

As the data come down from Mars, a computer program on Earth will scan each spectrum sent back by one of the MERs for telltale signs of rocks that signal the past presence of water on Mars.

"Of course, there’s sort of a holy grail here," Ruff says. "The best rock that we could find would be something like a carbonate or other sort of evaporite type of rock that on Earth we know, essentially only forms in the presence of standing water."

The Mini-TES’s specialty is to locate just that sort of rock among the much more common volcanic basalts that litter the landscape. If the Mini-TES detects an evaporite rock, the rock could be put on a short list of potential targets for a future rover excursion, one designed to search for signs of biological activity.

"We’ve done as much as we can with calibration and testing of those [Mini-TES] instruments and they’re off to Mars now." -Steve Ruff, ASUCredit: NASA/JPL

"The Mini-TES data analysis won’t be left entirely to the computer," Ruff says. "In the case of the really exciting or interesting or provocative results, there will certainly be human intervention. We typically never take those results without putting some human eyeballs on them too."

What’s Next

"We’ve built up a pretty substantial library of minerals, a spectral library, already," Ruff says. "But there are some additional kinds of samples that we’re interested in that we’ve never measured here. One of those is ice in soil." Before the landing, he says, "We will try to make some measurements of icy soils."

While the scientific teams behind other MER instruments may be able to continue calibration, the Mini-TES team has sent its twins to college and can only hope they’re successful. "The two Mini-TES instruments are already en route, and we have no additional engineering model to continue to play with here on Earth," Ruff says. "So we’ve done as much as we can with calibration and testing of those instruments and they’re off to Mars now."